Gas sensor

ABSTRACT

A gas sensor includes at least two light source, projection optics and a light-reflecting chamber provided with at least one light entry opening. The gas sensor further includes a detector that cooperates with the chamber, by means of which detector light from the light source can be detected. The at least two light sources can each be projected on a light entry opening of the chamber by means of the projection optics.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage of International Application No.PCT/NL2004/000652, filed Sep. 20, 2004, which claims the benefit ofNetherlands Application No. NL 1024364, filed Sep. 24, 2003, thecontents of which is incorporated by reference herein.

FIELD OF THE INVENTION

The invention relates to a gas sensor comprising at least one lightsource, projection optics and a light-reflecting chamber provided withat least one light entry opening, which gas sensor further comprises adetector that cooperates with the light-reflecting chamber, by means ofwhich detector light from the light source can be detected.

DISCUSSION OF THE PRIOR ART

With such a gas sensor, which is known from U.S. Pat. No. 5,734,165, alight source is projected on the light entry opening of alight-reflecting chamber via a mirror. The light from the light sourceis deflected by a mirror grating in the light chamber and directed tothe detector. The light is analysed by means of the detector, and fromthe analysed light at least the gas concentration of a gas isdetermined.

Gases can be selectively detected by making use of an infraredspectrometer, for example, and of their specific absorptioncharacteristics in the infrared spectral range.

Absorption of the light takes place in the gas that is present in thelight path between the light source and the detector. The length of saidlight path depends on the gas that is to be detected.

The stability and accuracy of the gas sensor are influenced inter aliaby the mechanical strength, thermal drift, changes in the humiditylevel, fouling of the components in the light path between the lightsource and the detector, ageing of the light source and the detector,etc. These effects influence the light to be detected by means of thedetector and can thus lead to a deviation in the determination of thegas concentration of the gas to be measured.

In particular variations in the position of the light source withrespect to the detector, for example due to thermal effects, will leadto an uncontrolled change in the light to be detected by means of thedetector when using the gas sensor according to the aforesaid U.S. Pat.No. 5,734,165, which will lead to undesirable deviations in the gasconcentration to be determined therefrom.

SUMMARY OF THE INVENTION

The object of the invention is to provide a gas sensor which isrelatively insensitive to influences such as thermal drift, fouling,ageing and mechanical changes.

The relative insensitivity to thermal drift, fouling and ageing of thegas sensor according to the invention is achieved in that the gas sensorcomprises at least two light sources, which can each be projected on alight entry opening of the chamber by means of the projection optics.

In this way it is possible to use one light source as a reference lightsource, whilst at least one other light source is suitable for gasdetection, for example in cooperation with an associatedwavelength-determining element.

The relative insensitivity to mechanical changes is achieved in that thelight from a light source, which is projected on the light entry openingof the chamber by means of the optics, is reflected in the chamber anumber of times on average, as a result of which the light distributionis homogenised, as it were.

The use of projection optics makes it possible to use a relatively largelight source, which can be projected on a reduced scale on a relativelysmall light entry opening. In this way, too, the sensitivity tomechanical changes is reduced.

The use of two light sources and a single detector makes it possible toobtain a gas sensor that does not comprise any moving parts, whichrenders the gas sensor less sensitive to failure.

It has furthermore become apparent that light sources remain relativelystable with the passage of time, which contributes to the long-termaccuracy of the gas sensor.

In an embodiment of the gas sensor according to the invention, the gassensor is provided with at least two light sources, which can each beprojected on the same light entry opening of the chamber by means ofprojection optics.

Since both light sources can be projected on one and the same lightentry opening, the gas sensor will be even less sensitive to smallchanges in the gas sensor mechanics, and the light paths between thelight sources and the detector are substantially identical.

In an embodiment of the gas sensor according to the invention, awavelength-determining element is disposed between at least one lightsource and the detector.

This makes it possible to use light beams having different wavelengths,which function as a reference light beam and a light beam for measuringthe desired gas respectively.

In an embodiment of the gas sensor according to the invention, theprojection optics comprises at least one projection mirror.

The occurrence of light absorption phenomena associated with the use ofa lens is prevented by using a mirror.

In an embodiment of the gas sensor according to the invention, themirror comprises a number of segments, a first group of which segmentscooperates with the first light source whilst the second group ofsegments cooperates with the second light source.

In this way, the light paths of the light beams from the light sourcewill be substantially identical. In addition, the effect of ageing andfouling of the mirror will be the same for both light sources when sucha segmented mirror is used, and it is possible to obtain a symmetricalincidence of light in the light-reflecting chamber. This makes the gassensor relatively insensitive to changes in the gas sensor mechanics.

The segments of the first group preferably have a first focal point,whilst the segments of the second group have a second focal point.

In an embodiment, the segments of the two groups are evenly distributedover the mirror.

In an embodiment of the gas sensor according to the invention, the lightsources are disposed on the same side of the detector.

In this way it is possible to position the light sources relativelyclose together, so that the light beams from the light sources willfollow substantially the same light paths to the chamber. This reducesthe sensitivity to changes in the mechanics even further.

In an embodiment, the light sources are spaced apart by a centredistance in the order of the diameter of the light sources. This, too,achieves that the light beams from the light sources substantiallycoincide.

In an embodiment of the gas sensor according to the invention, thelight-reflecting chamber is of square cross-section, at least one sideof which cross-section has a dimension in the order of the dimension ofa light-receiving element of the detector or of the dimension of theprojection of the light source.

As a result of this shape of the light-reflecting chamber, an optimumhomogenising effect is obtained. In an embodiment, the chamber istapered, as a result of which the light is concentrated in the directionof the detector.

In this way, the light that is present in the chamber is reflected atdifferent angles on different positions of the chamber, as a result ofwhich the light intensity measured by the detector remains the same,substantially independently of the position of the light source that isprojected on the light entry opening.

Because the light exit opening of the chamber is disposed close to thedetector surface, the relative position is guaranteed, as a result ofwhich any mechanical changes will not affect this part of the sensor anddrift of the measurements caused by the light shifting over the detectorsurface is thus prevented.

The claims and advantages will be more readily appreciated as the samebecomes better understood by reference to the following detaileddescription and considered in connection with the accompanying drawingsin which like reference symbols designate like parts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the basic principle of a gas sensoraccording to the invention.

FIG. 2 is a schematic view of a first embodiment of a gas sensoraccording to the invention.

FIG. 3 is a schematic view of a second embodiment of a gas sensoraccording to the invention.

FIG. 4 is a schematic view of a third embodiment of a gas sensoraccording to the invention.

FIG. 5 is a schematic view of a fourth embodiment of a gas sensoraccording to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows the basic principle of a gas sensor 1 according to theinvention, which comprises a light source 2 and a detector 3. A lightpath extends between the light source 2 and the detector 3, in whichlight path a wavelength-determining element in the form of an infraredfilter 4, projection optics in the form of a lens 5 and alight-reflecting, channel-shaped chamber 6 are disposed. Thelight-reflecting chamber 6 is elongate in shape and has a light entryopening 7 on a side facing towards the lens 5 and a light exit opening 8on a side facing towards the detector 3. The light source 2 is projectedon the light entry opening 7 as the light source 2′ by means of the lens5. The chamber 6 comprises a number of light-reflecting walls 9, whichbound a cavity 10 in which the light from the light source 2 isreflected a number of times before the light exits the chamber 6 via thelight exit opening 8 and falls on a light-sensitive surface 11 of thedetector 3. The light source 2 emits light which is filtered by thefilter 4, so that only infrared light having a specific, desiredwavelength will be passed on in the direction of the detector 3, saidwavelength being dependent on the gas that is to be detected. Dependingon the concentration of the gas to be detected, which is present in thespace in which the gas sensor 1 is present, a certain part of theinfrared light will be absorbed. On the basis of the amount of infraredlight measured by the detector 3, the concentration of a CO₂ gas, forexample, in the gas that is present in the space is determined. This isimportant, for example in order to determine the quality of the gas in aspace in which persons are present, such as a living-room, an office,etc. Said gas is the air that is present in the living-room. Since thelight path in the chamber 6 is relatively long, the light in the chamber6 is homogenised relatively well, as a result of which the light path isrelatively insensitive to mechanical changes.

The chamber 6 is preferably channel-shaped, the length preferably beingat least three times greater than the cross-sectional dimension so as toensure that the light in the chamber 6 is properly mixed. Preferably,said cross-sectional dimension is of the same order as the dimension ofthe light-sensitive surface 11 of the detector 3. Commercially availabledetectors 3 have a light-sensitive surface 11 of a few squaremillimetres. The walls 9 of the chamber 6 are preferably made of a metalthat reflects light well, for example gold, so that an optimumreflection is obtained. The cross-section of the chamber 6 is preferablysquare-shaped, for example oblong or square. Preferably, the chamber istapered in the longitudinal direction of the chamber 6, thecross-sectional area near the entry opening 7 and the cross-sectionalarea near the exit opening preferably differing from each other by afactor of up to 5.

FIG. 2 shows a first embodiment of a gas sensor 21 according to theinvention, which comprises two light sources 22, 23, filters 24, 25disposed in front of the light sources 22, 23, projection optics in theform of curved mirrors 26, 27 disposed in front of said filters 24, 25,a chamber 28 and a detector 29. The chamber 28 is provided with twolight entry openings 30, 31, on which the light sources 22, 23 areprojected as light sources 22′, 23′ by means of the mirrors 26, 27. Thechamber 28 furthermore comprises a light exit opening 32 centrallydisposed between the light entry openings 30, 31, opposite alight-receiving surface 33 of the detector 29.

The filters 24, 25 each transmit infrared light of a differentwavelength. If the presence of CO₂ is detected by means of the sensor,the wavelength for the filter 24 will be 4.3 μm, for example, and thewavelength for the reference filter 25 will be 4 μm, for example.

By means of the light source 22 and the associated filter 24, lighthaving a wavelength at which CO₂ absorbs maximally is transmitted, whichlight is directed towards the light entry opening 30 via the mirror 26,after which the light is reflected in the cavity bounded by the chamber28 a number of times before the light reaches the light-sensitivesurface 33 of the detector 29. By means of the detector 29, theconcentration of CO₂ gas that is present in a gas (air) surrounding thegas sensor 21 is determined. Light at which CO₂ absorbs minimally istransmitted by means of the filter 25, which light is subsequentlyanalysed in the same manner by means of the detector 29. The lightsource 23 functions as a reference. If a change occurs in the lightbeing emitted by the light source 23 and being detected by the detector29, this is an indication that a change has occurred in the gas sensor21, such as a decrease of the sensitivity of the detector, fouling ofone or more components of the sensor, etc. Subsequently, the measuredchange can be taken into account in the determination of the CO₂concentration by means of the light source 22. Preferably, the lightsources 22, 23 are alternately turned on and off at short intervals, sothat only the light from a single light source 23 or 24 needs to beanalysed by means of the detector 29. It is also possible, however, touse a detector by means of which the light from both light sources 22,23 can be simultaneously measured and analysed.

FIG. 3 shows a second embodiment of a gas sensor 34 according to theinvention, which is different from the gas sensor 21 that is shown inFIG. 2 in that the two light sources 22, 23 are projected on the samelight entry opening 35 of a chamber 36. The chamber 36 is provided witha light exit opening 37 on a side remote from the light entry opening35, opposite which a light-sensitive surface 33 of a detector 29 isdisposed. Compared to the gas sensor 21, the gas sensor 34 has theadvantage that practical identical light paths are formed.

FIG. 4 shows a third embodiment of a gas sensor 41 according to theinvention, which is different from the gas sensor 34 that is shown inFIG. 3 in that a faceted mirror 42 is provided instead of two mirrors26, 27, the mirror surfaces 43 of which reflect the light from the lightsource 23 in the direction of the light entry opening 35, whilst themirror surfaces 44 reflect light from the light source 23 in thedirection of the light entry opening 35. Ageing and fouling of thefaceted mirror 42 will occur to the same extent for both light sources22, 23, so that the gas concentration as determined on the basis of thelight source 23 can easily be corrected by means of the reference lightsource 23.

In addition, a symmetrical incidence of light on the light entry opening35 is obtained by means of the faceted mirror, resulting in an optimisedeffect of the channel.

FIG. 5 shows a fourth embodiment of a gas sensor 51 according to theinvention, which is provided with a detector 29, a light-reflecting,channel-shaped chamber 36, a segmented, faceted mirror 54 and two lightsources 22, 23, in front of each of which light source 22, 23 a filter24, 25 is disposed. The channel-shaped chamber 36 is of squarecross-section, with the cross-sectional area of the chamber 6 decreasingin the direction from the mirror 54 to the detector 29. The lightsources 22, 23 are disposed relatively close together on the same sideof the chamber 36. The mirror 54 comprises four segments 57, 58, 59, 60,which are more or less symmetrically distributed over the area of themirror 54. The segments 57, 59 form a first group of segments, whichcooperate with the light source 22, whilst the segments 58, 60 form asecond group of segments, which cooperate with the light source 23. Thetwo groups of segments have two different focal points.

The light beams 61, 62 from the light sources 22, 23 are projected onthe light entry opening 35 of the light-reflecting chamber 36 via thesegments 58, 60 and 57, 59, respectively. Subsequently, the light ishomogenised in the channel-shaped chamber 36 and analysed by means ofthe detector 29.

The distance between the light sources 22, 23 and the mirror 54 dependson the gas that is to be detected and the concentration thereof.

It is also possible to use three or more light sources, which makes itpossible to detect different gases with a gas sensor.

It is also possible to provide the faceted mirror with fewer or withmore mirror surfaces.

It is also possible, of course, to use other gases than CO₂ by means ofthe gas sensor according to the invention, with the filter 24 forexample transmitting light having a wavelength of 4.64 μm for CO or 3.4μm for HC.

It is also possible to project the light source on an enlarged scale, onthe same scale or on a reduced scale on the light entry opening.

While the invention has been described and illustrated in its preferredembodiments, it should be understood that departures may be madetherefrom within the scope of the invention, which is not limited to thedetails disclosed herein.

1. A gas sensor comprising at least two light sources, projection opticsand a light-reflecting chamber provided with at least one light entryopening, which gas sensor further comprises a detector that cooperateswith the chamber, by means of which detector light from the light sourcecan be detected, wherein the at least two light sources can each beprojected on a light entry opening of the chamber by means of saidprojection optics, and wherein the cross-sectional area of the chambergradually decreases from the light entry opening in the direction of thedetector.
 2. The gas sensor of claim 1, wherein the projection opticsproject each of the light sources on a reduced scale on a light entryopening of the chamber.
 3. The gas sensor of claim 1, wherein the atleast two light sources can each be projected on the same light entryopening of the chamber by means of projection optics.
 4. The gas sensorof claim 3, wherein the light paths between the light sources and thedetector are substantially identical.
 5. The gas sensor of claim 1,wherein the projection optics comprises at least one projection mirror.6. The gas sensor of claim 5, wherein the projection mirror is faceted.7. The gas sensor of claim 5, wherein the mirror comprises a number ofsegments, a first group of which segments is used for projecting thefirst light source on the light entry opening whilst the second group ofsegments is used for projecting the second light source on the lightentry opening.
 8. The gas sensor of claim 7, wherein the two groups ofsegments have two different focal points.
 9. The gas sensor of claim 1,wherein the light sources are disposed on the same side of the chamber.10. The gas sensor of claim 1, wherein the light sources are spacedapart by a centre distance in the order of the diameter of the lightsources.
 11. The gas sensor of claim 1, comprising at least three lightsources.
 12. The gas sensor of claim 1, wherein the chamber is of squarecross-section, at least one side of which cross-section has a dimensionin the order of the dimension of a light-receiving element of thedetector or of the dimension of the projection of the light source. 13.The gas sensor of claim 1, wherein the length of the chamber is at leastthree times greater than the cross-sectional dimension of the chamber.14. The gas sensor of claim 1, wherein the chamber is channel-shaped, atleast one dimension of the chamber being in the order of a dimension ofa light-receiving surface of the detector.
 15. The gas sensor of claim1, wherein the chamber is provided with a light exit opening, near whichlight exit opening the detector is mounted.
 16. The gas sensor of claim1, wherein a wavelength-determining element is disposed between at leastone light source and the detector.
 17. The gas sensor of claim 16,wherein the wavelength-determining element is a filter.
 18. The gassensor of claim 16, wherein the wavelength-determining element isdisposed between the light source and the projection optics.
 19. The gassensor of claim 16, wherein the wavelength-determining element isdisposed between the projection optics and the detector.
 20. A gassensor comprising at least two light sources, projection optics and alight-reflecting chamber provided with at least one light entry opening,which gas sensor further comprises a detector that cooperates with thechamber, by means of which detector light from the light source can bedetected, wherein the at least two light sources can each be projectedon a light entry opening of the chamber by means of said projectionoptics, and wherein the projection optics comprises at least one facetedprojection mirror.
 21. A gas sensor comprising at least two lightsources, projection optics and a light-reflecting chamber provided withat least one light entry opening, which gas sensor further comprises adetector that cooperates with the chamber, by means of which detectorlight from the light source can be detected, wherein the at least twolight sources can each be projected on a light entry opening of thechamber by means of said projection optics, wherein the projectionoptics comprises at least one projection mirror comprising a number ofsegments, a first group of which segments being used for projecting thefirst light source on the light entry opening whilst the second group ofsegments is used for projecting the second light source on the lightentry opening.